Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with agraphical user interface showing our new design.

The even length dashed broken lines are for environmental purposes onlyand form no part of the claimed design.

The uneven length dash-dot broken lines of the display screen representan unclaimed boundary line and form no part of the claimed design.

The text, “bpm,” “calories” and “rr”, and position of said text, is partof the claimed design.

CLAIM The ornamental design for a display screen or portion thereof witha graphical user interface, as shown and described.